1. Field of the Invention
This invention relates in general to catalytic apparatus for treating gases and, in particular, to improved structures of this type, together with methods of manufacture thereof, which are especially adapted for use with exhaust gases of internal combustion engines.
2. The Prior Art
As is well known, the exhaust gases emitted by internal combustion engines include noxious components which are susceptible of being converted into harmless gas components by means of catalysis. Typically, this conversion is achieved by introducing appropriate catalysts into the exhaust system. Such catalysts are generally in the form of an active coating on a carrier material or member.
Catalysis on an active surface is a function of the extent of contact with the active surface of the gases to be converted. An increase in active surface area results in a corresponding increase in chemical conversion capacity. Thus, it is desirable in exhaust gas purification to provide a catalytic unit or structure that affords as large an active surface area as possible. For most efficient use of the catalyst, the ratio of active surface area to weight of the catalytic unit should be as high as possible. Furthermore, as the chemical conversion of noxious substances by way of catalysis generally occurs only at high temperatures, the catalyst selected, as well as the unit in which it is incorporated, must be sufficiently resistant to high temperatures. In applications involving motor vehicles, such catalytic units must in addition be capable of absorbing impacts without modification or destruction of their structure. Accordingly, the prerequisites of a suitable catalytic unit, especially if it is to be used with motor vehicles, are a large active surface and resistance both to high temperatures and physical shock.
Various forms of catalytic devices have been proposed in the past. One type includes a carrier structure comprised of a number of corrugated plates which are stacked in such a way as to leave free hollow spaces between them. An active catalyst is applied to the carrier plates as a coating and the exhaust gas to be purified is passed through the spaces between the corrugated plates. Catalytic converters of this type may be designated as monolith units inasmuch as the several coated, corrugated plates are combined to form a single package or structure. This package is enclosed in a housing which is provided with intake and outlet openings for passage of the exhaust gases. An elastic, heat-resistant separator is provided in surrounding relation to the catalytic structure to absorb dimensional changes thereof caused by temperature fluctuations, thereby assuring that the package is securely held within the housing during use.
As size is an important consideration, especially when the unit is to be used in a motor vehicle, known monolith catalytic devices must be provided with the thinnest possible carrier plates so that a maximum surface area may be obtained in a minimum volume. At the same time, the carrier material must have sufficient structural strength and heat resistance properties to withstand the thermal and shock loads attendant with motor vehicle usage. This necessitates the use of costly materials. Currently available materials, however, still do not possess the required structural characteristics. Consequently, catalytic units of the foregoing type remain susceptible to shock loads and, after only a comparatively short time of use, fissures appear in the corrugated plates. As the number of fissures increases, parts of the coated corrugated plates break off and drop to the plate below. The result is a dense caking of the catalytic agent and thus a reduction in the active surface area, with consequent loss of catalyst function. The useful lifetime of such monolith structures in motor vehicles, therefore, is quite short.
In another type of prior art device, the carrier material takes the form of free-flowing, discrete particles, with the active catalytic agent being provided as a coating on the particles. An enclosure having intake and outlet openings is filled with these coated particles. The particles are held in place by grates or screens to enable passage of the exhaust gases through the enclosure without loss of the catalyst. Since exhaust gases are emitted from an internal combustion engine in a pulsating manner, the loose, coated particles are caused to assume a to and fro movement within the enclosure. The individual particles, however, do not move with the same speed and frequency, so that the individual particles rub against one another. This frictional rubbing results in an abrading away of the active coating from the carrier particles. Continued removal of the active coating reduces the active surface area available for catalytic reaction, and thus produces a consequent decrease in the ability of the catalyst to function adequately. Hence, although catalytic units of the loose particulate type are otherwise generally satisfactory for motor vehicle use, this abrasion problem so seriously shortens their lifetime that they become inoperable after only a comparatively brief period.
These and other shortcomings of the prior art are overcome by the present invention.